A theoretical interpretation of the dimuon invariant mass spectra measured in the acceptance of the NA60 spectrometer is shown in Fig. REF .
As a consequence of the subtraction scheme the spectrum is assumed to contain essentially only thermal radiation and any contribution from \(\rho \)  decay other than thermal, most prominently from \(\rho \)  mesons at chemical freeze-out.
The spectral distribution is successfully modelled assuming contributions from a hadronic, pion dominated thermalized fireball.
The respective yield is calculated by integrating the emissivity of a hadron gas at variable temperature and baryo-chemical potential over a conjectured space-time evolution of the collision zone.
The latter is adjusted such as to describe properly a number of hadronic observables like pion rapidity densities and transverse momentum spectra (see [1]} for a detailed description of the model).
The results which incorporate the in-medium spectral function of the \(\rho \) , obtained from hadronic many-body calculations and discussed in Sec. REF , is shown in Fig. REF  as solid (blue) curve (right panel, labeled Rapp/Wambach).
Also shown are three further curves to illustrate the discrimination power of the data against other hypotheses: expected yield assuming free (unmodified) \(\rho \)  decay with \(\rho \)  multiplicities derived from \(m_t\)  scaling (cockt. \(\rho \) ), spectral distribution assuming free \(\rho \)  decays but with a yield normalized to the measured yield in the \(M<0.9\)  G\(e\) V/\(c^{2}\) mass range (Vacuum \(\rho \) ) and ditto but assuming a shift of the \(\rho \)  pole mass according to a conjecture (Brown/Rho \(\rho \) ).
It is important to mention here again that the spectrum is shown in the acceptance of the spectrometer.
This is important as it was demonstrated that the acceptance filter of the NA60 spectrometer roughly cancels the Boltzmann term in the evaluation of emissivity based on VDM (see e.g. REF  ).
Hence, the result essentially represents the spectral distribution of the in-medium \(\rho \) .
The yield below masses of 0.6 G\(e\) V/\(c^{2}\)  therefore reflects \(\rho \)  strength dominantly caused by a coupling of the propagating \(\rho \)  to baryon-resonance hole states.
We will come back to this observation in Sec. .
The obvious mismatch of the shifted \(\rho \)  mass distribution with data has been taken as clear evidence for the absence of a pole mass shift due to a depletion of the chiral condensate as suggested by Brown-Rho scaling.
